A high voltage power conversion system typically consists of a controlled rectifier which connects to a utility power source and an inverter which connects to an output load. Traditional high voltage power conversions, including rectification and inversion, use phase-controlled current source converters which have several disadvantages such as requiring custom developed converter transformers, producing high levels of harmonics, drawing large reactive power and incurring high system cost. Implementation of an ordinary multilevel converter may eliminate the need for the transformer and thus reduce the system cost. However, the elimination of the transformer produces an undesirable voltage unbalance problem between the capacitors spanning the potential of the entire DC link. One proposed solution to this voltage unbalance problem is to replace the capacitors with batteries thereby making the potential at the respective converter levels a constant, however, this is not a practical solution due to limited battery life.
When connecting two adjacent AC power system grids, commonly referred to as back-to-back intertie, the frequency and phase angle difference between the systems can cause unnecessary circulating currents between the power systems resulting in significant power losses. The power loss commonly attributed to AC power system grid intertie can be as high as 10 percent to 20 percent of the overall available system power. When such losses are applied to a power system having an overall system power level of several hundred megawatts, a 10 percent to 20 percent power loss is unacceptable for practical and economic purposes. Due to the high losses associated with AC power system grid intertie, most back-to-back intertie systems use a DC link to avoid frequency and phase difference anomalies. Typical DC link back-to-back interties employ current source converters to convert power from AC to DC or vice versa. The 12-pulse type current source converters are quite common.
A typical 12-pulse converter for power system back-to-back intertie and high voltage DC power transmission consists primarily of two six-pulse converters having a 30 degree phase difference between device switchings. A phase shift transformer is required between the three-phase power system and the two converters to obtain the 30 degree phase shift. The obvious disadvantage of this circuit topology is the prominent harmonic content caused by square wave operation and reactive power components implemented for phase control. For a typical six-pulse converter, the AC current comprises approximately twenty percent of the fifth harmonic and fourteen percent of the seventh harmonic. For a typical 12-pulse converter, the AC current contains approximately nine percent of the eleventh harmonic and eight percent of the thirteenth harmonic. Consequently, the voltage at the DC side of the respective converters also contain significant even numbered order harmonics resulting in high power losses, acoustic noise, communication interference and heat stress of passive components and generators. A significant amount of filter circuitry is thusly necessary to attenuate the harmonics and the subsequent effects.
A modern converter station for power system intertie and high voltage DC transmission normally utilizes two sets of double-tuned AC filters and two sets of double-tuned DC filters to alleviate undesired harmonics. These converter stations normally require converter transformers between the AC line service and the 12-pulse converter. The converter transformers are specifically designed to have higher leakage inductance and to avoid saturation due to converter generated harmonics. Implementation of the converter transformer greatly increases the system cost as well as lessens system efficiency and reliability.
There are several solutions for the elimination of the converter transformer and harmonic filters for power system DC intertie. A multilevel voltage source converter is a conceivable option. A multilevel voltage source converter synthesizes the voltage waveform using DC link capacitors instead of the aforementioned AC transformers. Typically, a multilevel converter consists of several levels of capacitors on a DC bus. However, the capacitor voltage in a multilevel converter can not be balanced unless the capacitors are replaced by a constant DC voltage source. Due to size and cost constraints batteries are not a practical alternative.